text:
Why do buses at some airports differ from conventional ones?
Why do buses at airports differ from conventional ones?
Why do they have such wheels or cover wheels at all?

Source.

Many airports use standard-looking buses (but they tend to have more doors, fewer seats); Google Images suggests that these enclosed-wheel buses are fairly common at airports, though.

I've seen these buses as minimum in 7 airports where I was, and they were not big international airports

You said "Why do buses at airports differ from conventional ones?", which implies that buses at all airports are different. They're not. A better question would be "Why do buses at some airports...", with a particular focus on why the enclosed wheels are suitable/better for airports but not suitable for roads.

Very similar to this museum:)

Also note a buses operate in a common street/avenue cannot afford the luxury of that "low wheel" low floor. Since they operate in very different enviroments with very different envelope it's natural they are different

@RogerLipscombe The question "Why do X do Y?" is very commonly taken to mean "Why do some X do Y?" For example, "Why do people play sport?" is taken to mean "Why do the people who play sport do that?" rather than "Why is it that everybody plays sport?"

Apron buses are dedicated to transportation in the airport area, the one your show is located near the air museum at Salzburg airport / LOWS) in Austria. Similar buses are very commonly seen at airports:

Apron bus. Source.
The bus is traveling on short distances, it's maximum speed is 40 km/h, the price is lowered in proportion of the simplified design (more technical data).
Regarding the hidden wheels, considering the speed and the apron flat surface, this is possible for aesthetic reasons (many additional advantages come for free, e.g. fuel savings, protection against debris and stowaway). More importantly, the bus is a kneeling type type designed to accelerate passenger transfer. The floor can be lowered to:

To board and leave the bus more rapidly (no or small step).
To be accessible to everybody with no help, including when seated in a wheelchair (no lift required).

Protecting the wheels also keeps clear the space needed to lower the bus, and prevent accidents. This may be of some particular interest in a region subject to important snow falls, like in Austria.
Wheel access is provided by a panel:

Aero ABus
But this type of buses also exist with some or all apparent wheels:

Source.
Airport buses may be wider than regular road buses, they are not subject to public road regulation:

Source.
The model shown in the question can carry 100+ passengers in standing position (5 seats).

Source.
Apron buses can also have a bridge included (see also mobile lounges):

Source.

Unrelated comment: Salzburg, in addition of being the birthplace of Mozart and a famous ski resort, got an airport well known for its difficult approach during winter snowstorms, with the aircraft flying close to the mountain sides.

Washington DC Dulles (IAD) airport had, at least up until a few years ago, giant buses that looked like some 1970's vision of the buses of a moon colony.

Source: Wikipedia
The reason for this was that the buses articulated up several feet to reach and dock with the terminal buildings and sometimes with aircraft themselves.
I remember that they were slow and felt rather weird to ride - almost entirely unlike riding a downtown city bus.

Here are some reasons:

These buses have different transportation requirements. Mainly, they don't need to drive at high rates of speed, but rather carry large number of passengers over short distances.
The reasons the wheels are covered is to reduce the possibility of FOD (Foreign Object Debris) to get inside the wheels and force a repair on the tarmac; note that this not the standard design - many of these busses have exposed wheels.
The large windows, lower ingress/egress and limited number of seats are all designed for quick loading / unloading of passengers.


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What will create a "traffic puck" on the UAT band?
I know that for ADS-B in the US to receive TIS-B information you need to be a participating aircraft (ie ADSB-Out).  My question is more specifically, if you are flying an aircraft with a 1090ES transponder and have a stratux or similar ADSB-In device for your phone or tablet, will the system create the traffic puck around your aircraft and send TIS-B information for you?

Good question. As I understand it most 1090 MHz transponders transmit in dual-band mode, meaning 1090 and 978 (UAT) MHz. I'm more familiar with general aviation though, not necessarily the high altitude stuff.

@pugz most, if not all 1090 MHz transponders broadcast ADS-B only on 1090 MHz. I am not aware of any dual band transmitters.

@DeltaLima fair enough... I just know a lot of the general aviation ones that have 978 do both.

The answer is "yes" if
a) you configure your 1090 out system to request ADS-B uplink at 978 MHZ (this feature was added to the message structure in DO-260B)
b) your 1090 ADS-B out message is confirming certain minimum "quality" standards related to both the quality of the position information (GPS Integrity or "NIC") and the quality of the avionics (SDA or system design assurance)and the quality of the install or SIL  The specifics of what quality message triggers the system is under review by the FAA, but if you equipment is certified (TSO'd) and the install is in accordance with AC-20/165B you will be fine.
Note, if you have a UAT ADS-B you can specify which uplink frequency you want in the same way.
Further note - almost all ADS-B system are single band transmitters (ADS-B Out) and dual band receivers (ADS-B In).
Nuance - the uplink is broadcast to the quadrant (relative to the ground station) where you are flying.  This can lead to aircraft with no or "below quality" ADS-B out receiving traffic intended for another aircraft.  Depending on geometry this can create a partial traffic picture which can be quite misleading and potentially dangerous

No, you will most likely not get a TIS-B service dedicated to your aircraft.
The TIS-B transmitting system creates a 'traffic puck' around aircraft that:
a) have a version 2 ADS-B transmitter,
b) transmit the proper quality indicators which indicates that the system is certified and is properly functioning, and
c) indicate in their ADS-B transmissions that the aircraft is equipped with ADS-B IN. (see FAA ADS-B FAQ #34, link provided by @Rigged4Flight )
Typically TIS-B will provide information on the same frequency as the ADS-B transmitter, but the ADS-B protocol allows for indicating a different technology is used for ADS-B IN. I am not sure whether the FAA system uses this information.
Note: currently version 1 ADS-B aircraft with sufficient data quality are also still eligible for TIS-B services, however this will change in the next couple of years.

Thank you for the information.  I was aware of facts a and b but couldn't find anything on C.  Could you post a reference before I mark your answer as accepted?

Regarding DeltaLima's answer "C" (and Kevin's clarification question): DeltaLima is correct. The FAA FAQ on ADS-B says "To receive all ADS-B In services from the ADS-B ground infrastructure, aircraft must broadcast valid ADS-B Out messages that indicate their ADS-B-In capability."
https://www.faa.gov/nextgen/programs/adsb/faq/#34

@Rigged4Flight thank you for the info; I have included it in the answer.

@KevinJablonski answer is updated to include a reference (provided by R4F)

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text:
What's the relation between cabin pressure and altitude?
Usually, cabin pressure of an airliner is kept at a value corresponding to an altitude of not more than about 2000m.
Now, I heard that before takeoff, the pressure is already increased a little for higher stability of the fuselage. Maybe, this is wrong, but I had another experience:
I was in a 737-800 flying at 11500m. While the aircraft sank on approach for landing, I felt that typical pressure on my ears several times though we still were above 7000m. So, the cabin pressure already changed at that high altitude.
So, how is cabin pressure related to outside pressure during the entire flight?

Now, I heard that before takeoff, the pressure is already increased a little for higher stability of the fuselage. May be, this is wrong, but I had another experience: You are correct . Before rotation or in simple language just before take off, modern aircraft are pre-pressurised. This is to avoid surge in the cabin pressure.

@user20680 Depending on what you mean by "modern aircraft", this is untrue in at least some cases. All pressurized aircraft (all built in the 90s I believe) that I have flown have dump valves actuated by the gear squat switches, meaning without a system fault the aircraft cannot be pressurized on the ground. Proper pressurization control starts pressurization in the initial climb, and returns ΔP to 0 on short final. Other aircraft likely differ.

Here is a plot of cabin pressure on one flight. You can see a small increase in pressure right before takeoff. The decrease and increase in pressure is fairly constant during climb and descent. The pressure will change based on the aircraft's altitude, and will not reach a minimum limit until closer to the aircraft's service ceiling. This keeps pressure changes as slow as possible while giving a lower cabin altitude when below the service ceiling.

Source

On a number of aircraft types the cabin pressure is increased during the take-off roll, ensuring there is a slight over pressure when the aircraft rotates. This prevents some sudden changes in cabin pressure during rotation when the cabin exhaust valve is exposed to changing airflows. The chart in @fooot's post clearly shows the initial increase in pressure during take-off.
During the climb the cabin pressure is slowly decreased. While a typical passenger jet may climb at speeds in excess of 2000 feet per minute, the cabin pressure will decrease at an equivalent rate of climb rarely exceeding 500 feet per minute. When the cabin pressure has dropped to an equivalent of about 7000 ft altitude (the aircraft will be much higher by that time) the cabin pressure is kept constant.
When the aircraft starts to descent on approach to the airport, the cabin pressure is slowly increased at about a rate of 300 feet per minute. By the time the aircraft has reached the airport level, the cabin pressure should have reached ambient pressure. To achieve this, pilots have to set the cabin altitude for landing, otherwise the pressure will not equalize.

To the best of my knowledge, cabins are never pressurized before takeoff. This is a safety issue as it would inhibit door opening. The pressure would make them impossible to open.
Depending on aircraft, the aircraft might be kept at either:

At a constant pressure difference, i.e. the cabin altitude increases evenly up to cruise altitude. This would indeed mean that the moment it takes off it starts to build up a pressure in relation to takeoff. This means that the pressure difference is perceived as gradual.
At a fixed minimum pressure, i.e. the valve just closes at (for instance) 2000m and then maintains 2000m altitude all the way up, just building an increasing pressure differential.

Both methods have to take into account the landing altitude, since you  don't want to land the aircraft pressurized.
The former is more common since it's nicer. This would also explain why you perceived a change even at high altitude.

This answer says "The pre-pressurization occurs during the takeoff roll and shortly before landing." Thus, some aircraft's cabin are (slightly) pressurized while on ground.

Some cabin doors open outward, in which case interior pressure would actually assist in opening the doors.

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text:
If airplanes can't use reverse thrust in flight, what am I hearing?
I've flown on a few commercial flights, and on several of them I recall a phase of the flight during descent when the engine seemed to go idle (or at least, the engine noise subsided substantially) and then all of the sudden, the engine got really loud and I could feel the plane slowing down substantially, as in, I felt the same as if I was in a car that was slowing down suddenly.  This seemed to last between 5 and 15 seconds, or possibly slightly longer, and then the engines got more quiet and the noticeable deceleration stopped.
Living under an incoming flight path for a few years I have also noticed many times that a plane will be going overhead by the fact that suddenly the engine will get few loud for about the same amount of time.
This seems to be rather common, and I always assume this was reverse thrust.  But today I am reading two separate questions: "Is it possible to use reverse thrust while airborne?" and "Why can't most jets safely use reverse thrust while airborne?", the answers to both of which seem to imply that using reverse thrust while airborne is very rare.
So what have I experienced / heard?  I'm pretty sure it's not an airbrake, because far as I can tell a speed brake is a more passive device that doesn't involve the engine, and it would seem to defeat its purpose to spool up the engine while using it.  I've never been in a seat where I could actually see the back of an engine to tell if a reverser was deployed, but I think at least in some cases the speed brakes were also deployed; from in front of the wing this made it impossible to see the back of the engine.
Update: Everyone is saying speed brakes... how long are these usually deployed for?

FWIW, on 747-100 and -200 aircraft it is theoretically impossible to use reverse thrust in flight if things are working as they should. There's air or ground logic that has to show the aircraft is on the ground before the reverse levers are unlocked.

@Terry: How well did the theory hold up in practice?

@Vikki In this instance as I remember, theory and the real world match. I don't recall a 747 having a thrust reverser deploying in flight. That in itself doesn't mean that such hasn't happened since I'm 82 and have some dementia.We occasionally would have a reverser not deploy when it should have, but even that was rare. It never happened to me.

It is normal for engines to spool up during the approach. The initial part of the approach, from cruise level down to approximately 10 miles from the airport, is flown at flight idle power. This is the most efficient way to get down. In the final part of the approach, flaps and landing gear add so much drag that the engines need to be well above idle power.
At low speeds required for landing, the wing of the aircraft will not produce enough lift to sustain flight. To compensate for the low speed, flaps are extended. They do not only increase the lift, they also increase the drag. When the flaps are extended, suddenly the drag increases, resulting in a deceleration. Extending the landing gear has a similar effect.
Normally the flight crew will limit the time that they fly at low speed and high drag. The first couple of degrees of flaps deployment are relatively efficient;they do not add a lot of drag but allow the aircraft to fly slower. When the aircraft is about 7 to 10 nautical miles from the runway, the crew will transition the aircraft to landing configuration for the final approach. Often there is a horizontal segment before the aircraft starts a 3 degree glide path down to the runway. The crew will ensure that enough flaps (drag) is added before they start this final segment, otherwise they have difficulty reducing speed (they usually try to prevent use of speed brakes in that part of the flight).
Often there is a sequence of flap and gear extension before the final descent segment starts. This causes the aircraft to decelerate from the efficient high speed configuration (flaps 2 or less on the B737) to the landing configuration. At the end of that configuration sequence the aircraft has a lot of drag and is decelerating rapidly towards the final approach speed. To stop the deceleration extra power is commanded which spools up the engines and stops the deceleration. To reduce the spool up time of the engines, the engine control system (FADEC) will give a short burst of power. This what you hear while still decelerating. When the engines are spooled up they start to produce thrust which is the end of your deceleration.
This youtube video shows how it happens. When at 28:20, the aircraft (Boeing 737) is flying horizontally, flaps 2, and is about to start decelerating and descending for landing. Keep a look at the thrust levers(right bottom corner of the screen) and the airspeed (on the left of the artificial horizon).
It begins at 28:28 when the captain ask for flaps 5, and reduces the thrust in order to reduce to final approach speed. 15 seconds later he asks for gear down and flaps 15. The speed is now reducing rapidly. Starting at 28:50 the thrust levers are moved forward, until they are reduced again at 29:08. This is the power boost you hear. In 40 seconds the aircraft is transformed from low drag / low thrust to landing configuration.

Excellent explanation, thanks.  This sequence of steps is very helpful in understanding exactly what is going on when.

@Michael the only conflict is your feeling that deceleration stopped when the engine spools down again which is not true - the deceleration actually resumes or increases. I bet that's all that is - just your feeling: an illusion due to your expectation that noise = something happening.

The noise you are most likely hearing from inside the cabin is the speed brakes (also known as spoilers). In addition to drag, they create a lot of turbulence, which creates a lot of noise. After looking at some videos, it doesn't seem to come through on video as well. There does seem to be more noise at lower frequencies though. The FAA Airplane Flying Handbook notes:

There is usually a certain amount of noise and buffeting associated with the use of speed brakes...

When outside the aircraft, you may be hearing the engines spool up. During approach and landing, the pilots are extending flaps and landing gear. This and the maneuvering can cause the autothrottle to spool up the engines to compensate for added drag, or the pilots may be manually adjusting the throttle, to maintain or reach a certain speed. Speed brakes are generally used when the airplane is higher and faster on approach, and when heard from the ground, the plane is more likely to be lower and slower, using throttle to adjust speed.

interesting... so the noise is loud enough to be heard from the ground?

Not sure about speed brakes... I'll edit that in.

ok thanks, so I may be hearing two separate things.  The weird thing about hearing it from the ground is that it seems to only last for a few seconds... I would expect if it was engines spooling up they wouldn't spool back down so fast.  But maybe, like you said, it's just doing some compensating.

My guess would be that you're feeling accelerations more upwards rather than to the rear. It is common when flying a plane to need to adjust descent rate for various parts of the approach, either because different phases of the approach actually need different descent rates or to correct your glide path. When you need to decrease your rate of descent (or even level out) for a part of the approach, you do that by temporarily adding engine power. This will also usually be accompanied by pitching up a bit. I'm guessing that the forces you're feeling are the descent rate slowing down rather than the actual forward speed slowing down (though you might be doing some of that, too, if they pitch up enough.) I've experienced this commonly on commercial flights during approaches. We also make use of the engine to slow a descent/correct a glide path when flying light aircraft. Once the desired descent rate has been achieved, engine power will be reduced again, so this results in the few-to-several-second-long bursts of engine power that you're describing.
As fooot mentioned, when the gear and flaps are dropped, engine power is also added to compensate for the increased drag in order to maintain airspeed. However, this is not generally just a brief burst of power as in the above case of needing to arrest a descent rate.
As others have mentioned, there is some noise when spoilers (speed brakes) are being used to quickly slow down the aircraft in flight, but, at least in my experience, this sounds nothing even remotely like the engines spooling up. I would certainly not expect that you'd hear this from the ground.
It is, however, rather certain that you are not feeling thrust reversers. As far as I know, these are never intentionally deployed in flight on current commercial airliners (as UFO mentions, the DC-8 was actually certified for that, but I doubt you hear many DC-8s flying over these days.) There have been a few cases where malfunctions have caused them to deploy in flight, but I don't remember hearing about any cases of them being intentionally deployed in flight on current-model airliners.

Decent rate slowing feels different... that's more of a pushing down in the seat.

@Reirab -- DC-8 pilots use in-flight reversing both for normal and abnormal/emergency procedures, as the DC-8 lacks flight spoilers/speedbrakes.

@Michael You'll feel some of both when this happens because of the changing pitch.

@UnrecognizedFallingObject Interesting! I've never flown on a DC-8, so I wasn't aware of that. I'm fairly certain that no current airliners do that, though.

@UnrecognizedFallingObject - as did Concorde

Quite not possible because of the nature of reverse thrust (may not be as directional and controllable as required.)
  Can't say of future developments, but as of now, spoilers are the maximum possible "Airborne Braking" that can be deployed.

Well, I'd say pitching up is an even more effective form of 'airborne braking.' :) Using spoilers along with it helps to prevent undesired altitude gain, though.

If you weren’t flying in a Dc-8(which could reverse in flight on no 2 and 3), you are probably just feeling the level-off and thrust increase to keep the speed and present flight level/altitude during the stepdown descent of a normal approach..
That might be mistaken as a deceleration, with no forward view, by a passenger. A pilot would understand it differently. I am not saying anyone’s stupid, just that with a pilot education and a few thousand hours, the perspective will be different..

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text:
Why are visual and ILS approach angles for a Cessna 172 often different?

NOTE: I fly in Canada. Also, the question is based on my experience on light aircraft like a C172.

It is known that the (standard) glideslope flown in an ILS approach is 3°. However, that doesn't seem to be the case for a visual approach. Some basic high school math tells us that an aircraft 1 NM away from the aiming point on a 3° ILS glidepath would be at 319 FT AGL, whereas I'm taught to be at around 600 FT AGL (or, "ideally" 500' AGL- the same as when turning crosswind) when turning on (approximately) a 1 NM final from base in a VFR traffic pattern- making my approach path [2] about 4.7° - 5.6°.
What is the reason for this difference in glideslope between a visual and ILS approach?
An answer at this PPRuNe post says that the angle was chosen as it supposedly worked well for the flight computer to be able to AUTO LAND the aircraft. However, that begs the question: why enforce this even in a CAT 1 ILS approach where the pilot takes manual control (once visual) way before touchdown?

Why do you think you should fly such a steep final in a VFR pattern?  I was trained to roll out on final exactly on the VGSI, where one exists.

"whereas I'm supposed to be at around 600 FT AGL when turning on (approximately) a 1 NM final from base in a VFR traffic pattern- making my "glideslope"[2] about 5.6o." According to whom? Please provide a reference.

@Pilothead According to all the instructors at my school. The airport elevation is (rounded off to) 200’, so we’re asked to be at (about) 800’ at the beginning of final. I will cross check tomorrow and get back to you.

@Pilothead I Cross-checked with my instructor who says the ideal height at 1NM final is 500' AGL, and another student whose instructor says approx. 600' AGL. Added an answer based on the conversations.

Are you sure you want to always put yourself in a situation where you are 100% reliant on the engine to make the runway during VFR flying in a light airplane?  I was taught to have the power at idle during VFR final approaches in single-engine light aircraft.  This results in a glide path steeper than the ILS glide slope.

@quietflyer And as I alluded to earlier, that works fine if your plan is to fly primarily light, low-inertia, high-lift airplanes like a 152 or 172.  Energy management in an airplane only 500 or 1,000 pounds heavier and 5 or 10 kts faster is like a whole different world.

@quietflyer Yes, I have heard that being recommended as well. Although it's customary to not do so on a "normal" approach, I think no one's gonna mind it if I do (especially because it's recommended and GOOD practice). I recently gave my PPL Flight test and my examiner also recommended keeping the flaps to 20 until closer to the RWY and just let the plane "GLIDE" at 1500 RPM, so we don't have to make too many power changes.

Understand that your glide slope through the air also depends on the headwind component. The greater the headwind is, the shallower your glide path through the air is compared to the glide path angle relative to the ground.

Several factors and thoughts here.

I think you might be flying a very light aircraft with very slow approach speeds that support steeper approaches, also meaning less vertical descent energy to be absorbed in the round out/flare.  As you move to heavier/faster aircraft you’re going to need to be staying fairly close to 3 degrees.  In my aircraft a descent steeper than 3-4 degrees is going to absolutely require one of two things.  I’m either going to be flying the final well below 1.3x V(s) or I am going to float a long ways and need a very long runway, like literally 1,000 more feet.

Remember that VASI/VGSI systems are most usually also 3 degrees.  Turning in from one NM at 600 feet is putting you well above the VASI visual slope.  Again, that may be fine for a slower/lighter aircraft but if you’re flying something like a 182 or Bonanza, it’s really, really hard to get that plane on the ground unless you are very deliberate about matching the right speed to the right descent angle.

I very much disagree with your quote PPRuNe quote about auto land.  The 3 degree convention for ILS distantly predates any modern auto-landing systems.

Thanks for the answer Yes, you're right- I fly a Cessna 172. I have been trained to not look at the PAPI lights but try to achieve the "right picture of the RWY" while judging height on approach. Apart from that, I take it from your answer that the 3 degree angle was decided to make the NAVAID universally usable by all aircraft types (since both a C172 and a B747 can approach t a 3 degree glideslope), right? I haven't done any instrument approaches in the C172 yet, so I don't have a lot of understanding at this point.

@PapaMike99 Remember that landing is the ultimate energy management maneuver.  I have to bring my vertical energy to zero at touchdown and my horizontal energy to zero before the end of the runway.  A light aircraft at 60Kt GS is descending at 600 FPM in order to go from 600 AGL to 0 AGL.  600FPM x 1900 Lbs (vert) and 60Kts x 1900 Lbs (horz) is well within the control authority available in a 172 on 2,000 feet of runway.  600FPM x 600,000 Lbs (v) and 150Kt x 600,000 Lbs (h) is a profoundly different problem to solve, the technical term for these kinds of energy is “an ass-ton,” LOL.

@PapaMike99 a loaded 747 would need a flare longer than the entire runway to fully arrest a 6 degree descent at 150-ish Kts.  At some point, aeronautical engineers, landing gear structural engineers, runway strength civil engineers, land planners who had to buy runway land, found 3 degrees to be about the right compromise of safe approach speeds, reasonable flap and elevator sizes, reasonable brake design, reasonable land use, etc.

makes sense for an airliner. But don’t Cessna’s also approach at 3 degrees on ILS? If so then why? 3 degrees would put a 172 much lower than what we’re taught here in BC, Canada!

@PapaMike99 it would be impractical to have a ground-based ILS system configurable to specific aircraft, and creates the possibility of the airport being mis-configured for the landing, not the airplane.  We already have non-precisions approaches using WAAS GPS that would allow you to determine your own vertical profile at or above the minimum altitudes.  But by not relying on a ground-based ILS, you become subject to higher minimums.  So if you wanted to fly in steeper than the ILS, you’d request the GPS approach and manage your own altitude, within the limits of the published approach.

it's been a while. I added a new answer based on my increased understanding of flying since I asked this question.

Training to not look at the PAPI lights but try to achieve the "right picture of the RWY" sounds absolutely unwise to me. The "right picture" for one runway will look quite different than being at the same point for a runway with different length, width, slope, terrain, and so forth.  Ignoring the standard, objective, constant, repeatable info from a PAPI in favor of "seems to look right" isn't the way that any training I've ever see does it, and I'd be very wary of such philosophies. Do we ignore the precision of an ADI, an airspeed indicator, a VVI, etc for such reasons???

@RalphJ I get your point. And we weren’t exactly told to “ignore” the PAPI, rather to develop our own judgement to prepare us for runways without a PAPI.

There is actually a very good reason to approach at 6 degrees in a 172:  more of a chance you make the runway if the engine fails.
This is true for all single engine aircraft.
The 172 has a glide ratio of 1 to 9.  Arc sine 1/9 = 6.4 degrees.
Keep in mind a headwind will shorten gliding distance.  Flying an approach at 65 knots in a 172 puts one right at Vbg.  A little high on base leg is even safer.  This is where learning to forward slip is very useful.
Although landing under power is easier (RPM can be "fine tuned" with throttle), there is no chance making the runway at 3 degrees approach unless the engine is running.  Multi-engine aircraft have at least one extra in reserve.

Is the engine more likely to fail during the final approach, be it 3 degrees or 6 degrees, than during the rest of the flight between when you take off and when you get to that final?  If the concern is to be in the best position to make the runway should an engine fail, how is that concern met for the other 95% of the flight?

@RalphJ Save takeoff and climb, with the "other 95% of the flight", one has potential energy in altitude to glide to a safe place.  A single engine on a 3 degree final will crash short of the runway if the engine is lost.  Where I learned to fly there was a 2 story building that we had to clear to land on the runway.

OP's answer[1]
As I've learned after going through my flight training and further discussions with lots of people (and reading the comments and other answer on this post), the visual approach path MAY or MAY NOT be 3° depending on what is being done. The following factors (and perhaps more) come in to play:
The aiming point
When visual, one can aim at the RWY THR, 1000' RWY length marker or even before the THR. The ILS glidepath is designed to make the 1000' marker the aiming point. Therefore, depending upon how far away the visual aiming point is from the marker, the visual approach path may closely or may not at all align with the ILS glide path.
Efficiency
An ILS approach is flown at a higher speed and lower flaps setting than a visual approach (just to keep the extra drag away in case a missed approach has to be done). A C172 is usually flown at 90 KIAS without flaps and the speed is only reduced to 60 KIAS and about 20° of flaps extended once visual - at an altitude that can be as low as 200' AGL. When approaching visually, the final approach/landing configuration is set up much before. To maintain a shallower descent angle with a higher flaps setting, a higher amount of thrust is required. This need can be eliminated through a slightly higher descent angle.
Operational needs
A VFR pilot may choose  a steeper approach path and an aiming point further upwind of the RWY THR to provide for altitude in case of an engine failure.
Room for error
Lastly, the most obvious factor is that it is very hard to eyeball an exact 3° glideslope and, moreover, considering a margin of error in the attempt of maintaining a reasonable descent rate, it makes sense to be a tiny bit higher than a tiny bit lower. Getting lower and lower often leads pilots - especially new ones - to end up dragging in the approach, sometimes even below VREF.

Now that I think of it, I think the last two are the reasons why my school's policy was to teach students to do it the way as I've mentioned in the qeustion.
Bottom line is, a 4° - 5.5° degree glideslope isn't unreasonable considering the basic nuiances of visually hand flying an aircraft. That being said, a person can definitely get skilled at eyeballing an approach path very close to 3° and maintaining a good descent rate with practice. At the end, visually manoeuvering an aircraft just broadens the spectrum of what can and cannot be done and also the required buffer for error.

[1] I deleted my previous answer - even though it received positive votes - as it was far more inaccurate compared to this one.


text:
Is it okay to hear the stall horn on a small aircraft while flaring?
Original Question: Is it okay to purposely stall a small single-engine aircraft just before touch down to get a good flare?

At the time of asking this question, I didn't know a myriad of things about actual flying that I do today, and this question branched out from confusions due to lack of knowledge. However, I think that the main question about the onset of the stall horn remains valid (regardless of the impractical manoeuver through which I got to it). So I sought to edit this post to make it less nonsensical to future readers rather than deleting it.

I'm still learning ground theory and have only flown in simulators (P3D v3 Academic). For a long time, I've been trying to properly flare single engine aircraft but have mostly failed. I even watched a video on youtube made by a certain flight instructor that said that the smaller airplanes like a Cessna 172 and Mooney acclaim don't flare - they "transition" just to make sure the rear gears touch down first.
What I did to flare better in the SIM was: as I approached the threshold, I gradually started pulling the throttle and pitching up. In that condition, the airplane would tend to sink rather than go up and since the aircraft's height wasn't much above the ground, the impact from a "fall", I thought, would not be fatal.
However, I thought that I was essentially stalling the aircraft (purposely) as I even got a stall warning (though it lasted for about 2 secs). What I was trying to do here is making sure that the nose wheel was higher so the mains would touch down first.
As far as I know, a stall situation is usually a bad one. However, I was thinking that it might be okay if the pilot knew what they were doing and still had control while (kind of) stalling the aircraft. But I'm confused since I haven't flown a real bird yet.
So, my question is, Is it okay to hear the stall horn on a small aircraft while flaring?

Related: What is the best method to time your round out or flare in Cessna 172?

@papamike99  You may wish to invest the time flying with an instructor.  There are "short field" techniques, coming in slower (and more nose high). However, gusts and wind gradients can make doing this extremely dangerous.  "Rounding out" involves pulling the elevator, which can stall the aircraft (and crash it). Much safer to (for example) approach 65 knots, round at 65 knots (10-15 feet off runway), flare down to 50 knots (stall warning comes on), settle onto runway.  You don't have to stall to settle, just reduce AOA (pitch) slightly.

Anecdotally, I often hear the stall warner just before touchdown in a PA28 or C172. Its not really much to worry about in those sort of trainer aircraft. The horn goes off a good few knots before the wings actually stall and the nose drops.

@RobertDiGiovanni Thank you for the helpful words. I will soon go to a flight school and, at present, am simply trying to make sense of what I learned theoretically, in the simulator.

I do prefer not to hear the stall warning on takeoff though, especially when I am not the PIC.  Makes me a bit nervous every time it happens.

"To get a good flare".  Never heard of pilots talking about "wow, I had a great flare on that landing!", although a very famous test pilot really had a beauty landing the XF-92A, reducing landing speed from around 160 to 67 mph.
So why do we flare?  What is a flare?
"Flaring"  is increasing AOA to MAINTAIN lift as the plane slows down.
Why do we slow down as much as possible before landing?
Too fast and the plane will "bounce". Bouncing is not a major problem if it is on the main gear (and not too hard).  Bouncing the nose gear can be disasterous.  Since you have a lower AOA for the same lift when faster, risk of bouncing the nose gear is greater.  For taildragger, the risk is prop strike.
So you flare to increase AOA after rounding out and prior to touchdown, reducing speed and increasing AOA.
Do you need to stall to get a good flare?  NO!
Does the stall warning mean I'm stalled?  Try it at altitude first.  Generally they are set a few knots ABOVE stall, and it is normal to hear it as you touch down.  Know your plane.
Finally, "round out" at approach speed (you don't want to stall here), then reduce throttle and pitch up (flare).  The trick is to hold it a few feet off the runway as speed decreases and AOA increases.  You are now flaring!
Actually landing, well, everyone has their own ideas.  In a 172, I could just start to feel the seat of my pants drop.  At that point, back pressure on the yoke was relaxed just a tiny bit (slightly reducing AOA and lift), and the plane would settle onto the runway.
Flaring is all about speed control.  Keep it safe.

Thank you for the answer, Robert. I see that the priority is not to get a good looking flare but to have a steady approach and touch down (and making sure the rear gears touch down first). Just one thing though: what exactly do you mean by "round out"?

"Round out" is bringing the plane level (stopping your descent) a few feet over the runway, prior to flaring and landing.

To prop strike a taildragger you have to be going really really fast.  When flying TDs I prefer to "wheel land" them, tail high, and when you do that you actually fly down to a foot or so above the surface, applying just enough pitch to stop the descent, then instead of holding the nose up with increasing elevator, you PUSH to plant the mains, and keep pushing to hold the tail up until you slow down.  This affords way more control in gusty conditions and xwinds.

No mention made of relationship between ground speed in excess of stall speed, and landing roll distance. This is probably the most significant reason we try to slow down as close to the stall speed as possible before landing. Done right, this happens as close to the approach end of the runway as is safely controllable, so that we have the maximum amount of runway remaining to stop.

@CharlesBretana not always a good idea, especially in wind.  This question is from a student who may benefit from not trying for short field perfection at that stage of training.  Flaring at a safe approach speed provides precious seconds to set up a good landing.  This is why learning with a lot of excess runway is very helpful.

@robert, of course!  We often forget that landing at an airfield where you need to touchdown in the first 50 feet means your departure  from that field will be on a truck to a field with a longer runway. That is why we teach short field landing practice to aim for a spot significantly down the runway. But the op's question, to me was about why we might want to slow the aircraft down as much as possible before touchdown. And the answer to that should at minimum mention the relationship between speed and landing roll distance required.

I've talked about good and bad flares, with pilots and students. Often.

In my experience the best altitude for a fully stalled landing is about 1 inch. Seriously though, the method I use is to fly as close as I can to the ground without touching down with practically no throttle and finally reducing the remaining throttle whilst maintaining the height above the ground by increasing the angle of attack until it finally stalls and drops that last few inches onto the ground. It's difficult to get right but if you land prematurely it's not the end of the world, but if you use too much elevator too soon you will balloon (increase height) and you don't want to stall from there, apply a bit a power and try again.  I would suggest paracticing by flying as close to the runway as you can without touching it, using throttle and attitude to remain just above the stall, using the end of the runway as your reference (v. important), climb out, do a circuit and trying again until it 'click's.  It all depends on the aircraft and wind conditions, but if you're flying a light aircraft without a significant cross wind, this is pretty much the best way and eventually you'll be able to maintain a few inches just above the stall and then close the throttle to settle fully stalled onto the ground.  If it's done right your stick/control column should be fully back.  As I say this is my experience and I know there are people who would vehemently disagee, but it works for me.

Just to be sure, you do this in the real world or in the sim?

That's a bit unnecessary PapaMike99.  I fly real aircraft in the uk and have had a pilot's licence for about 20 years.  I don't fly that regularly admittedly but I'm giving my opinion in good faith, take it or leave it.  I have never used a sim - I don't play computer games at all and I'm close to retirement age.

Phil, I am extremely sorry if my comment sounded disrespectful. I am simply looking forward to gaining knowledge and just wanted to know how much real world influence your knowledge had. I absolutely appreciate you taking the time to answer my amateur question; it's just that I am quite confused right now after reading various opinions from different people, so am a little obsessed with verifying the authenticity of statements at the moment. I once again apologize and sincerely thank you for your answer. :-)

I had considered being "vehement", but easing out the last bit of throttle before settling is a good idea. Throttle control can really "fine tune" things, as it's control is more precise. Good answer!

I have had several such "drop an inch" landings as a passenger on LOT Polish Airlines in the 80s and early 90s. They were especially memorable on AN-24 where you could see the wheels floating so close to the runway that their shadow was mostly obscured by the wheel, and then you could see the struts compressing relatively slowly. IIRC it was mostly done in good weather and runway conditions, and not all pilots did it.

I'm training in a Cirrus SR-20 which is a small single engine a/c. The way I was taught was to start rounding out and pulling power simultaneously, roughly 10-20 ft. above the ground. You want to flare to about 5 degrees pitch up and ideally hear the stall horn as the mains touch the ground. I should add also that when you hear the stall horn, at least in the SR-20, it is not indicating a full stall, in fact it is just indicating an incipient stall, i.e., you are about to stall. Again, this is just how I was trained, and this is something my flight school really stresses on.
Personally, I believe the aim is to be pitched up enough so that you aren't landing flat on all 3 wheels, and so that you've lost enough energy to prevent the a/c from bouncing once you do touch down. You want to be slow enough and losing enough lift that the aircraft wants to land. But the key is that you're  not doing this when you're still 20 ft. above the ground. You should only be a few inches off the ground when you reach your flare attitude and hear the stall horn. And you want to hold that attitude until the mains are both down and you can't hold the nose up anymore.
Good rule of thumb is to fly the airplane down to the ground. Do not stop "flying" your aircraft until you have fully landed. Hope this was helpful!

I practised flapless landings with a Saab Supporter, flying the aircraft unto the runway and controlling speed only by the throttle till touchdown. I kept the aircraft low and slow and must hear the horn to flare which stalls it unto my aimpoint. Errors in height judgements caused hard landings. But the aircraft was strong and I had leeway in flying as it was a military airfield.

Hearing the stall warning during landing is how you know you’ve squeezed the very last ounce of lift out of the wing and are landing at the absolute slowest ground speed possible in the current conditions. Except in instances of strong crosswind or soft field conditions, you want to hear the stall warning horn on every landing.  20+ years of flying, I can only manage it 1 out of every 5 landings.

Depends on the airplane and type of landing gear it’s equipped with, and the toe of landing you are performing, but in general, stalling the airplane during the roundout is NOT recommended.
It is true that, while performing a three point landing in a tail wheel airplane, you will touchdown on all three wheels in a stalling attitude. And the aircraft essentially does stall about 1 to 2 feet above the ground, thence plop down on all three wheels.  But this applies only to a tail wheel airplane and only a specific kind of landing. You don’t stall a tailwheel airplane on a wheel landing, and he will not do so on a tricycle gear airplane, either.
Remember that a roundout (flare) is performed to accomplish two things: 1) arrest your rate of descent on approach as you near the ground and 2) place the airplane in the correct attitude so that it gently makes contact with the ground on its landing gear.  This requires the pilot to fly a final approach to the runway at an airspeed Vref either recommended by the manufacturer, but not less than 1.3*Vso.  Flying this airspeed on final approach allows the airplane just enough kinetic energy to accomplish the two required tasks of the roundout before being exhausted and settling onto the runway without excessive floating and using up too much runway in the roundout.
Exact techniques vary a little between airplane to airplane, but typically for light airplanes with tricycle landing gear, you will fly your short final stabilized at Vref towards your aimpoint for starting the roundout (not the touchdown point).  If you have not achieved a stabilized, and trimmed up approach at Vref by short final, you should execute an immediate go-around, re-enter the pattern and try again.  At approximately 10 to 20 feet above the runway, power should be smoothly reduced to idle, and aft stick pressure should be applied to arrest you descent so you end up in level slow flight approx 1-2 feet above the runway.  Height above the ground should be judged by perspective alone, not the appearance of the runway to avoid narrow runway illusions.  Thereafter you will use aft stick pressure to bring the aircraft into the correct touchdown attitude, and hold it in that attitude until the airplane settles onto the runway. If you were on the Vref on short final, the aircraft should use up ~400 ft of runway in the roundout, so pick you aimpoint some 400 or so feet before your desired touchdown point. The instant touchdown occurs, you transition from slow flight into a high-speed taxi.  Gently release aft stick pressure following touchdown to gently lower the nosewheel onto the runway, then apply brake pressure to slow the airplane down to safe taxi speed prior to exiting the runway.
Due to the more rigorous demands on the landing gear during a soft field landing, you will touchdown as smoothly and as gently as possible on these types of landings.  In a soft feel landing, you may will hear the stall warning horn, but only a fraction of a second before the airplane makes contact with the runway, not sooner.  Short field  landings are a totally different animal.  The same basic principles apply in a short field landing as a normal landing, but touchdown at a desired point on the runway, while consuming minimal distance in the flare is required.  These types of landings will not be greasers, although a hard or excessively rough landing shows bad flare technique.  You want to plop it on the ground and keep it down as quickly as you can, but do so following the correct landing technique and not so hard it could damage the airplane.
Jet airplanes are typically flown onto the runway, as opposed to doing some sort of hold off until just short of stall.  The reason for this is to prevent the airplane from consuming an excessive amount of runway during the round out and jets are aerodynamically pretty slippery without a windmilling propeller attached to them slowing them down.

thanks for including energy management in your answer!


text:
How is optimal Glide Path Altitude calculated?
Good day,
On a PPL checkride quite a while ago I got asked "if you are 10 miles from the airport how high should you be for a 3 degree glideslope?" Can someone run me down on how to do the math to figure it out quickly?

The quick math would be the reverse of the calculation needed to calculate the number of miles needed to descend from one altitude to another. Take 10, subtract 2, divide by 3, then multiply by 1000. This gives you close to 2,700 feet.

Isn’t HEIGHT of aircraft more useful information on the glide path? Technically, if you’re asked to find ALTITUDE then you have to take QNH into account. The math will give you HEIGHT.

Just memorize the value 320 ft/nautical mile.
A 3 degree slope is a 19:1 gradient.  1 nautical mile is 6076 ft, divided by 19, = 319.8 ft.
Round up to 320 ft/mile which is close enough, and memorize that value, and multiply by the number of miles in your head as required, so 10 miles is easy; 3200 ft above the runway threshold.  Add to that the runway elevation and there's your target indicated altitude value.
If you're asked for an odd number like 7 miles, the quickest way to mentally work out 7 times 320, for me at least, is to to multiply hundreds first, then the tens after and sum them.  So, 300 x 7, for 2100, then multiply 20 x 7, or 140, and add 140 to 2100 to get 2240.  That type of mental math structure seems to work well when under pressure or with distractions.
For statute miles, the value is 280 ft/mile (actually 279, but close enough).

While @JohnK’s answer is more accurate, the usual rule of thumb is 3nm/1000ft to determine when to start your descent. This “pilot math” is much easier and faster to do in your head, and the error is within the variation due to wind anyway.
In your case, the question is reversed, so 1000ft/3nm, or 333ft/1nm.

THIS is by far the best answer! I can do "three miles per thousand feet" far faster & more accurately than "miles x 320 feet". If a delta of 133' at 10 miles matters, then you're waaay beyond the realm of rules of thumb!

In a long descent yes, and I use that, but his question was related to determining height on a glideslope closer to the airport, which you could take to mean what is the height above threshold of an ILS 10 miles out, where a bit more precision would be useful.

@JohnK Reversing a simple rule I use on every flight will get me an answer faster and easier than getting out a calculator to do it your way. Sometimes speed matters more than precision.

@StephenS The question wasn't about rules of thumb for when to start an enroute descent.  The question was more direct, in effect, what is the height above threshold on a 3 deg GLIDESLOPE 10 miles out, inferring an ILS glideslope, and demanding a more precise number than the 60:1 rule for enroute descents gives.  If the OP just said what gives a ~ 3 deg angle on an enroute descent, I would have given your answer.

It is very simple. You can get your distance on a 3 degree glide by multiplying the altitude by distance. For example, if you want to know what your distance is at 3000 ft, just divide the altitude by 1000 and then multiply the number you get by 3. In this case (3 × 3) = 9 nm.
If you want to know what your altitude is at a certain distance then you can reverse the calculation. Divide the distance by 3. So, 10 nm is  (10 ÷ 3) = 3.3. Multiply this by 1000 which will give you an altitude of 3300 ft.
You can also use trigonometry. That is a triangle with a 3 degree angle with 10 nm as adjacent, we can find the opposite.
tan 3 = x/10
x = 10 tan 3
x = 0.524 nm.
You can then convert it to feet. The answer then becomes 3183 ft.
The first method I mentioned is a simple short cut that you can use in the aircraft (the most accurate method ofcourse is the trig).
It works from a simple Cessna to the largest jet. I still use the same calculation for the A320 and it works. But in jets we add about 10 nm to the answer for deceleration.


text:
How is an aircraft aligned at a jet bridge?
I see these jet bridges (or aerobridges) at nearly every new airport I go to. How are aircraft aligned with these bridges, particularly the A380 which has doors on both decks?

How are aircraft aligned with these bridges

They are not (but read on)
Aircraft are aligned to a point on the concrete, not to the jetbridge(s).
It is the jetbridge itself that carries out the final alignment to the aircraft door.
Jetbridges are actually motor vehicles with wheels, tyres, motors and (often) steering. It just happens that their rear end is tethered and their bodies articulate and/or telescope to accomodate the motion at the "front" end..
You drive one using a set of controls at the end furthest from the terminal building
-jet bridge controls
- driving a jet bridge
I believe some also have what is effectively variable height suspension like old Citroens (except I imagine it is hydraulic or jack-screw).
The aircraft is driven to the terminal so the nose wheel follows a  guide line and stops at a marking appropriate for the aircraft type. Then the jet bridge is driven up to the aircraft door.
As the aircraft is gradually burdened with self-loading freight and sinks on it's suspension, the jetbridge metaphorically bends its knees so that the SLF don't trip up or fall into the aircraft. That is, the jetbridge maintains alignment, the aircraft is pretty passive in the job of alignment.

I have to admit I didn't notice these jetbridge controls for many years as I was usually either busy rushing to my seat or busy rushing to the baggage reclaim. One time there was a delay at the aircraft door that was long enough for my bored gaze to alight on the controls and for the thought to briefly wander across my mind - "I wonder what would happen if I pressed that ..."

"I wonder what would happen if I pressed that ..." - the reason why there should't be allowed red buttons on control panels other than the terminate-stop-panic one

@Mindwin Or the one that announces how many times it's been pressed.

There are two red buttons one of which is 'stop' and the other appears to be 'stop I'm on fire' (or some other cause of smoke)

Auto-leveller.. that is what maintains the constant height relative of the jet bridge to the aircraft. There's usually a roller which folds out and contacts the aircraft fuselage. As the aircraft moves down (pax leaving the rear and coming forward) the roller will move and this will tell the bridge to lower.

I'm actually finding that I'm happier being "self-loading freight" than the ominous-sounding "pax".

The bridge can move to an extent but the aircraft is driven up to a designated spot that is marked on the ground. Here is a video of it all happening.

This can be also accomplished with someone on the ground directing

you cannot be directly connected with the ground crew while the aircraft is autonomously moving (and pushbacks are not used for gate alignment)

My apologies I will amend the answer

you edited too much, I'll restore the correct part :P

So after the jumbo jet is parked, does this guy goes back to his game of ping pong?

@Federico Actually, a pushback tug CAN pull the aircraft into the gate, although this is used only when necessary -- usually in cases of extremely tight wingtip clearance where very precise steering is required. When this is done, the aircraft is stopped on a mark, the engines are shut down, and then the ground crew hooks up the towbar and establishes communication in the same manner as used for pushing back. The pilots then release brakes, & the aircraft is pulled in to the gate.  Something similar can also be done if the aircraft is stopped & shut down too far off the mark -- not often.

I've been on a plane where they've used a tug due to snow. I think it was Stockholm. "We're just waiting for a tug to pull us up to the gate" comes over the PA. After some movement, and a halt, a further announcement: "we're just waiting for a larger tug..."

The Aircraft aligns with the gate through ground markings, radar sensors and eventually ground operators signals.
After the aircraft stops the jet bridge is aligned with the aircraft's door by an operator that commands the various bridge actuators (wheels, extension, rotation).

Image source

They align themselves with the stand guidance system
Azimuth Guidance for Nose-In Stand (AGNIS) is a passive system which uses a system similar to PAPI lights to display red and green lights to indicate whether the plane is too far left or right.
An active system is for example the Honeywell Advanced Visual Docking Guidance. A camera detect the airplane and changes the sign to guide the pilot to the correct place.

(image source wikipedia)
After the plane is aligned to the gate the jet bridge is maneuvered up to the plane's door.

this is the alignment to the gate, not to the "jet bridges"

being aligned to the gate is required for the jet bridges to reach the plane.

I agree, but aligning to the gate does not imply being aligned to the bridges. and the questio is "how the aircraft aligns to the bridges" not "to the gate"

Federico, I can't understsnd your comments.  Parking "at the gate" (alingng left/right, and distance wise as in the photo above with the yellow markings) IS INDEED, PRECISELY parking so as to align with the jetway.  Ratchet - am I right?  (Or is the "gate" something else?!  many yards back, or??)  Sure, the OP needs to additionally understand that the jetway moves on wheels.


text:
Does temperature affect takeoff performance?
A few days ago, a Copa flight from Porto Alegre to Panama was delayed from noon until 12:30 AM. The company explained that the high temperature (about 40 degrees Celsius) would require a longer runway for takeoff since the air was thinner, and that resulted in non-operational conditions for the wings and engines with the available runway length.
One source (in portuguese): http://www.radioguaiba.com.br/Noticias/?Noticia=515816
Is this true? Can a high temperature really prevent takeoff?

There is also the problem of maximum fuel temperature for some aircraft. As I remember for 747-100 and -200 aircraft with Pratt & Whitney engines it was 54 C. I remember that being a concern only once when the aircraft had gotten heat-soaked sitting on the ramp in Bahrain on a really hot day for them.

I'm not going to post as an answer, because all the science is already well-enough given below. But this video shows what happens to airplanes with low density altitude: http://www.youtube.com/watch?v=yDu0jYiz-v8&oref=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DyDu0jYiz-v8&has_verified=1

Holy crap that's one hell of a video

Summer of 1990 we were awaiting some other students at a summer internship program.  They arrived a day late because they flew through Phoenix, AZ and the previous day it was so hot that they had to shut down the airport because the air was too thin to provide adequate lift for take off and landing.  See answers below for the technical stuff.

Yes, temperature effects airplane performance both the engine itself and the aerodynamics involved.
What Is Density Altitude?

Density altitude is pressure altitude corrected for nonstandard
  temperature. As temperature and altitude increase, air density
  decreases. In a sense, it's the altitude at which the airplane "feels"
  its flying.

How Will High Density Altitude Affect Flight?

On a hot and humid day, the aircraft will accelerate more slowly down
  the runway, will need to move faster to attain the same lift, and will
  climb more slowly. The less dense the air, the less lift, the more
  lackluster the climb, and the longer the distance needed for takeoff
  and landing. Fewer air molecules in a given volume of air also result
  in reduced propeller efficiency and therefore reduced net thrust. All
  of these factors can lead to an accident if the poor performance has
  not been anticipated.

Tips for Flying in High Density Altitude Areas
One way of looking at it is that higher temperatures make the airplane fly the way it would at a higher altitude where the air is less dense. An airplane cannot climb to infinity, it has what is called a service ceiling [and an absolute ceiling] that the aircraft can no longer climb any higher. This also because of a decrease in air density, but for a different reason. Raising the temperature of the air also decreases this air density. You can use an Air Density Calculator or a Density Altitude Chart:

to determine how the aircraft will fly. It is especially important at airports with short runways or high actual altitudes [Denver, Colorado for example] where the aircraft already may need to be near maximum performance.
If the density altitude is 4,000ft than that means that airplane will fly like it was at 4,000ft at standard atmospheric temperature. Standard atmospheric temperature [at sea level] is 15 degrees Celsius or 59 degrees Fahrenheit. An airplane departing at a sea level airport, at 100F would feel like it is already flying at over 2600ft. An airplane departing at a 5,000ft airport would feel like it is already flying at over 8700ft. If clearing the trees at the end of the runway was already a concern at this particular airport it may not happen anymore.

Can't give a much better answer to this question than this. :-)

Does the fact that they move slower in cold weather and faster in hot weather also affect the aircraft's performance or no? So if its cold there is a higher chance or air molecules hitting the wing and engines because the air molecules are compacted and slow moving and in hot weather they are spread out and faster moving, so less of a chance of air molecules hitting the wings and going into the engine.

Yes, it is quite correct that an airplane requires a longer runway on a hot day. The effect is the same as being at a high altitude. High up on a hot day, in the middle of a low pressure zone is the worst, as the effects are combined.
What it comes down to is air density, less dense air means less "stuff" for the wings to sail through, generating less lift, and thus requires higher airspeed before being able to lift off. On top of that, engines need oxygen to fuel the combustion, less dense air means less oxygen and therefore less power output from the engine. And just as the wings are less effective, so are the propeller or fan blades, converting less engine power to thrust.
So in essence, low density means you'll need more airspeed to climb away from the ground, and you have less power at your disposal for reaching that airspeed.
A well known video taped Stinson crash in Idaho a while back is generally attributed to high density altitude. High density altitude means low air density (may sound counterintuitive, but it basically means that it is equivalent of a high altitude, where the atmosphere is naturally less dense). The airfield elevation was 6370 ft, however the density altitude on that day has been calculated to have been 9167 ft, a substantial difference.

I love this video. Mainly because everybody walked away so we can show it over and over again to remind people. Not just about preflight planning but this guy had SO many chances to abort. I love watching this video with people and being like still could abort, still could abort, has plenty of room to land again, still plenty of room land, maybe turn slightly and stay over that ginormous field? No.. no.. you're just going for the trees. There are a couple of hazardous attitudes at play he is just going for it no matter what in the face of anything. He must have known he had a full 3 minutes.

@p1l0t; it's good learning video for sure. Being honest with myself, I think I'd make my decision to set it down right about the 2:10 mark of the video, still being level with and awfully close to the tree tops that far along in the takeoff. But who knows, it's hard to say one would react, I've found myself lower than I would've liked over trees (off the departure end at my local airfield), but I only really fully realized the climb performance was miserable (HOT!) when I was already over said trees.

There is a point of no return, but before that time I'm waiting for an excuse to abort. I'd rather feel silly about aborting than pulling pine cones out of my landing gears. Another good habit I have is I know that on a hot/humid day at max gross I can make a certain distance no sweat (say 2500ft) over a 50ft obstacle. That way I can say OK 6k is plenty or hey 2200ft I really better crunch the numbers on this. I also have the advantage of most airports I fly out of being near sea-level though. In the mountains if you're loaded and not face first in the charts then you're kind of asking for it.

@roe He definitely should have decided to abort. However, I will contend that he never should have had to make the decision to abort. This is simple pre-flight planning. And he failed it miserably. You never want to have to make the decision that you never should have had to make.

@Shawn; Oh definitely, no argument here. I'm just saying that it's easy to look from the outside and say "I'd never do that!". I'm a stickler for preflight planning, yet sometimes we still find ourselves behind the airplane, and we realized it later than we thought we would.

We are all capable, but this is why we hangar fly,  watch these videos, practice emergencies, read accident reports, and etc.. We can't possibly live long enough to make all the mistakes ourselves.

@p1l0t "We can't possibly live long enough to make all the mistakes ourselves." So very true. :-)

Absolutely: thinner air, whether caused by a higher altitude or a higher temperature, decreases aircraft performance in two ways:

It decreases the amount of air going into the engine, which means that less fuel can be introduced in order to maintain the correct fuel to air mixture. This produces less power output from the engine and decreases the thrust generated by the fans in the turbofan engines.  This increases the amount of time (and therefore runway distance) that it takes to accelerate to flying speed.
At a particular airspeed, there is less actual air (fewer air particles) moving across the wings.  Since this is what produces lift, the airplane must be moving faster in order to create the same amount of lift.  This requires more time, and therefore more runway.

Climb performance is also reduced for the same reasons, and can also be a factor preventing an aircraft from taking off if there are obstacles that must be out-climbed (there is also a minimum amount of climb required after takeoff for certification, so performance charts don't even have information past a certain point).
For a "real world" example, take a look at this chart from a jet that I fly, and notice that the hotter the temperature or the higher the takeoff altitude, the lower the weight that you are allowed to takeoff at:

At 40 deg C, even at seal level, this airplane is limited by climb performance to a takeoff weight of about 13,700 kg. (instead of the normal 17,600 kg.), and this doesn't even consider the amount of runway required!
The exact same principle applies to runway distance required, but there are additional variables which make the graph much more complicated to follow:

Thanks, got it. It's a big diff in fuel in a long flight...

@woliveirajr Yes, because of the limited takeoff weight, they may not be able to carry enough fuel to get to their destination and still takeoff with the runway available.

How do you read/interpret that second graph? Or is it something that's calculated by a specific formula?

@BurhanKhalid That graph is complicated to follow, and this particular example is harder because they start with two limiting factors (temperature and runway length) and see where they meet in the middle. The arrows guide you through it though: start at the top, draw a line from the current temperature to the right until it reaches the airport pressure altitude. Then draw a line straight down until it reaches the reference line for the next section. You then draw a line parallel to the existing lines down and to the right. Repeat from the bottom up (backwards) until it joins your first line.

@BurhanKhalid When working it backwards though, you don't go straight to the reference line. In this case you draw a line straight up from the runway length until it meets a line drawn from the left side, starting at the amount of wind (headwind in this case).  Then you parallel the existing lines until you reach the reference line. Repeat this until you reach your first line, and then draw a line to the left to see the maximum takeoff weight for the current temperature and runway length. If the runway was very long, you would just work tip to bottom and determine the runway required instead.

As others have said, yes it can and does happen.  In PHX a few years ago the airport was closed to most aircraft for temperatures at/above 120.  Note please it is not that the planes can't fly in that heat, it is because the planes don't have the proper high density altitude charts so they can't determine how long a runway is needed.  i.e., for part 121 operations such a flight would be illegal.
Why don't they have the performance tables?  It costs a lot to develop them, and IF they exist, the manufacturers sell them as an extra cost item to the plane's operator.
I don't believe there is any max temperature limit for the small GA planes that many of us fly, except to say that 120+ degrees is @#&*$# hot in a non-airconditioned GA plane.

You would still need performance numbers in a small airplane!

For a part 91 flight?  Yes, the prudent pilot would want that information, but is it legally required?  I know of no such requirement.  But I have been wrong before, so edumicate me please!


text:
What is the oldest aircraft still in production?
What is the oldest aircraft still in production? I Googled it but only got the answer to the oldest military aircraft still in production.

Do you mean oldest type with a continuous production run, or the oldest design still being hammered together?

@egid the answers to both would be interesting. ;)

@egid also, if production only paused for about ten years or so I would count it

Well, given aviation's occasional financial... issues, what is in production today might not be tomorrow. ;)

Shouldn't the Piper Cub be considered a candidate?  At least if you allow variations/improvements on a basic design...

Longest production run (civil)
The Beechcraft Bonanza seems like a likely candidate for 'oldest design still being made'; first flight was 1945, and it's still in production today (albeit made by the same parent company that makes Cessnas!). Even during the general aviation downturn in the US of the 1980s & 90s, production continued, as Kevin Cathcart pointed out in the comments:

FAA registrations show no gap in years of manufacture of the A36 Bonaza. I was able to find a registration for one manufactured in every year from 1980-1999 in california alone. Production did drastically slow during that time frame, but it never stopped.

Longest production run (military)
The Lockheed C-130 Hercules military transport first flew in 1954 and has been in continuous production ever since. According to Wikipedia, they've even been built in the same factory (in Marietta, Georgia) the entire time.

Because only around 2,300 have been produced (only!), they don't show up on the 'most produced aircraft list' mentioned in another answer.

FAA registrations show no gap in years of manufacture of the A36 Bonaza. I was able to find a registration for one manufactured in every year from 1980-1999 in california alone. Production did drastically slow during that time frame, but it never stopped.

Incorporated that into the answer. Thanks, I didn't even think to check the aircraft database!

The Great Lakes 2T1A biplane trainer was designed during the roaring 20s and first flew in 1929. While there were many gaps in production over the years as the company and plans changed hands, it is currently in production - a design close to 90 years old!

I don't know if it counts, but the Messerschmitt Me-262 has seen a second production run of five aircraft being finished earlier this century after approx. 1500 had been built before the middle of the last, from 1941 - 1945. The new production was started in 1993 by the Texas Aircraft Factory, led by someone who had learned metal aircraft craftsmanship as an apprentice in Germany during WW II and who later moved to the US. The job was finished in Seattle by ex-Boeing people, and the aircraft were rivet-for-rivet copies from the original design, with one exception: They are powered by J-85 engines instead of the old Junkers 004B. The Junkers jets were similar in size and mass to the EJ 200 of the Eurofighter, but with 1/7 of the thrust and just 50 hours between overhauls were considered a liability.
Already in 1950, in the early phase of the Korean war, a new series of the Me-262 was briefly considered when US aircraft found themselves to be outclassed by Soviet-built jets. In the end, the Me-262 wing design heavily influenced that of the North American F-86 Sabre.

A gander on the list of most produced aircraft on Wikipedia suggests the Cessna 172 which started production in 1956 and is still produced today.
However the Aeronca Model 7 Champion has returned to production in 2007 after a short production run from 1945-1950.

Does anyone know if the 172 has been in continuous production (i.e. at least one manufactured in each consecutive calendar year) since '56 - I assume it has.

@RedGrittyBrick "Production halted in the mid-1980s, but resumed in 1996 with the..."

@ratchetfreak yeah, same goes for the Beechcraft Bonanza, which is actually about 10 years older than the 172. Basically all US GA production ceased during the Great Legal Nightmare.


text:
What are the design goals for NASA's X-57?
There have obviously been work going on prior to the X-57 announcement (see LEAPTech and Joby Aviation http://www.jobyaviation.com/LEAPTech/). However, what does NASA actually think they will achieve as far as specifications after completing the X-57? I have seen the 175 mph cruise at 1/5th the energy cost numbers thrown around, but the real numbers that matter are not stated by NASA. For instance, will the entire aircraft except for the pilot seat be packed with batteries? What is the useful payload? What will be the range on a single charge?

I am leaving this as a separate question as the referenced question was specific to the type of wing technologies and not the design goals of the specific aircraft.

That the same answer is being used gives indication it's a duplicate.

I just found an answer (pointed out by @RoboKaren), on another question. The only reason I am copying this is that the original question was not really about goals. This is from Mark Moore:
source: What are the advantages of the NASA LEAPTech propeller on wing technology?

The lift augmentation has nothing to do with Magnus lift (or the
  Coanda effect). It is simply an increase in the dynamic pressure
  across the entire streamtube of the wing.
At the low takeoff and landing speeds (slightly above a 61 knot stall
  speed), the induced velocity of the propeller almost doubles the
  velocity that the wing sees; and the lift is a function of the
  effective velocity squared. But due to swirl and other effects the
  wing doesn't experience a 4x increase in lift, but about 2 to 3 times.
The objective of the inboard propellers is not to achieve a high
  propulsive efficiency, instead we want those inboard props to achieve
  high induced velocities - think of them as part of the high lift
  system (which happens to also provide thrust redundancy).
In fact landing is the critical case, and having lower propulsive
  efficiency (and a poorer spanwise lift distribution because of the
  swirl effects) is helpful in creating sufficient drag. The inboard
  props are not active in cruise flight, but simply fold against the
  nacelle (many motorgliders already do this type of folding on the
  nose).
By only using the wingtip propeller at cruise, we're able to achieve a
  ~95% propulsive efficiency (versus 75 to 85% with a typical fuselage
  nose propeller installation). The reason for this is because we have
  lower blockage and scrubbing drag, as well as being able to take
  advantage of the strong wingtip vortex by rotating against it. Since
  electric motors don't experience a power lapse with altitude (because
  it's non airbreathing) we have far too much power at altitude anyway
  so by only using the wingtip motor it doesn't cause much penalty in
  motor weight (and electric motors achieve ~6x lower weight per
  horsepower than a reciprocating engine).
In terms of batteries and range, the key is to achieve high efficiency
  cruise flight and it looks like with current batteries a 200 mile
  range is achievable. By adding a small <50 hp range extender motor,
  the aircraft will be able to achieve ~400 mile range. We're currently
  designing an X-plane that will fly in 2017 to substantiate all these
  numbers, with the ground test rig (wing and truck) providing an
  aerodynamic database to validate our analysis.
Please note that I'm the NASA Principal Investigator of the LEAPTech
  Distributed Electric Propulsion integration approach, and the
  Convergent Electric Propulsion Technology (CEPT) X-Plane demonstrator.
  We have a team across NASA Langley and Armstrong, as well as two great
  small companies, Joby Aviation and ESAero who are retrofitting a
  Tecnam P2006T with a Distributed Electric Propulsion wing system.

Please add the source for the quote

I had referenced the comment above with the source before, but I explicitly put it in the answer now. Thanks!


text:
How can I find traffic patterns that pass over my house?
I live in Atlanta, GA well north of the airport and typically there is very few large, commercial aircraft that pass overhead. But there is an on-going security issue at the airport today and there are some really low-flying commercial jets passing from east-west over my house at altitudes that look like they would be on final approach (if they weren't 40 miles north of the runways anyway). There is a military airport near by and we frequently have C-130's landing but they don't come from that direction.
So I really have no idea what those planes were doing or where they were going, and in the 5 years I've lived here there have never been commercial planes that flew that pattern. At another place I hang out, close to the airport, I've noticed that air traffic passes over when there are storms at the airport.
I would imagine this is a "something is wrong at the airport" pattern as opposed to a normal operating pattern, and it may even be something that was specific to that particular aircraft for some reason.
Is there any way to look up what patterns would pass over a given location under what situations? Or more likely -- how can I look up what patterns are around an airport and how those patterns change with conditions (weather, emergency, traffic, etc)?

Its not that easy for a non-pilot to do it, but there are about a dozen "Arrival Procedures" that show specific routes into ATL.
You can also see on the Atlanta Area Chart how some of the traffic might flow near your house.
The easiest thing to do, if you can pick out some of the aircraft, is to look up their flight paths on Flightaware:

You can also tune in to ATL approach and get more information as well.

Since the question is about how to find them, and not actually asking for a listing of them, I'll answer that one option would be to locate a flight school (ideally one operating near KATL like Aero Atlanta at DeKalb-Peachtree) and pay for a one hour ground school lesson specifically on how to read KATL's Visual and Instrument approaches (the flights could be on either depending on what's happening with weather and ATC.) A CFI would be able to help you identify which procedures cross low over your area.
The downside is it will cost you (probably in the neighborhood of $50) but you'll get a really interesting introduction to airport procedures. And may even lead to a discovery flight!
You may also find out that none of the arrival procedures involve low routes over you and ATC was vectoring traffic into unusual areas to create space for a situation at the field. Within Class B airspace, all aircraft are under positive radar control by ATC, meaning they can pretty much send any aircraft anywhere they want if the situation warrants and the pilots are able to comply. Still for noise abatement reasons they usually wouldn't have large jets flying low unless they were on an approach.


text:
The F-16s that were scrambled to intercept United 93 did not have time to arm the jets, so the pilots were going to “ram it” how does that work?
The Wikipedia page says

Two F-16 fighter pilots from the 121st Fighter Squadron of the D.C. Air National Guard, Marc Sasseville and Heather "Lucky" Penney, were scrambled and ordered to intercept Flight 93. The pilots intended to ram it since they did not have time to arm the jets; this was in the days before armed jets stood ready to take off at a moment's notice to protect the capital's airspace.[140] They never reached Flight 93 and did not learn of its crash until hours afterwards.[141]

Is this exactly what I think it means? Crashing into another aircraft? What I’m picturing is the fighter pilots coming in above and behind, and using the nose of their aircraft in either a downward or upward motion to eradicate the stabilizer and elevator completely.
Any info is greatly appreciated!

I can't see this linked anywhere in the answers, so I'll just link it here. Header did a TED talk: https://www.youtube.com/watch?v=2gOo08NMSrs

I'm reminded of the RAF tactic against V1 flying bombs of wing destabilisation: fly the interceptor plane very close, but not touching the wing surface of the target. The airflow disruption could "stall" one wing, and the primitive guidance system could not recover and crashed.

@Firefigther1, it's likely you're wildly over-reading a wiki page.  Even if the person who most recently edited that page, copied that text including the word "ram" from some official document, it's totally unclear what the full original context was.

Note that ramming has been a viable last resort air combat tactic for basically the entire history of air combat: https://en.wikipedia.org/wiki/Aerial_ramming (see the RAND study mentioned in 2nd last paragraph)

In a Hollywood action film the F-16 pilot would probably swoop in front of the jetliner and then use the afterburner as a weapon, drop an external fuel tank into one of the engines, or maybe deploy a drogue cute over the cockpit to blind the pilots...

I expect that an ejection seat would play a key role in this maneuver.

Just read the Washington Post article that was cited in the Wikipedia article.
The intention of the two F-16 pilots was to ram the Boeing 757 in a likely suicide mission, since they had no ammunition and no other armed aircraft were standing by:

The day’s fourth hijacked airliner seemed to be hurtling toward Washington. Penney, one of the first two combat pilots in the air that morning, was told to stop it.
The one thing she didn’t have as she roared into the crystalline sky was live ammunition. Or missiles. Or anything at all to throw at a hostile aircraft.
Except her own plane. So that was the plan.
Because the surprise attacks were unfolding, in that innocent age, faster than they could arm war planes, Penney and her commanding officer went up to fly their jets straight into a Boeing 757.
“We wouldn’t be shooting it down. We’d be ramming the aircraft,” Penney recalls of her charge that day. “I would essentially be a kamikaze pilot.”
[...]
In the monumental confusion of those first hours, it was impossible to get clear orders. Nothing was ready. The jets were still equipped with dummy bullets from the training mission.
As remarkable as it seems now, there were no armed aircraft standing by and no system in place to scramble them over Washington.

One of the pilots was going to ram the cockpit of the Boeing 757, and the other pilot was going to ram the tail. They didn't target the engine because the hijackers might still be able to guide the plane to their target:

“I’m going to go for the cockpit,” Sasseville said.
She replied without hesitating. “I’ll take the tail.”
[...]
“We don’t train to bring down airliners,” said Sasseville, now stationed at the Pentagon. “If you just hit the engine, it could still glide and you could guide it to a target. My thought was the cockpit or the wing.”

They also thought about ejecting out of their jets just before impact, but there was the risk of missing the airliner if they did so:

He also thought about his ejection seat. Would there be an instant just before impact? “I was hoping to do both at the same time,” he says. “It probably wasn’t going to work, but that’s what I was hoping.”
Penney worried about missing the target if she tried to bail out. “If you eject and your jet soars through without impact . . .” she trails off, the thought of failing more dreadful than the thought of dying.

Also notable is the fact that Heather Penney was willing to carry out the mission even though she knew it was possible her father was in the cockpit of Flight 93:

Later, as the Penney family checked in on each other from around the country, they marveled at the other fateful twist on the extraordinary events: the possibility that Penney’s own father could well have been in the cockpit of her airliner target.
John Penney was a captain at United Airlines at the time. He had been flying East Coasts routes all the previous month. The daughter had no way of knowing whether the father was airborne or not.

Would it be frowned upon to switch the answers? I feel like this answer and the one below it answers the question best?

@Firefighter1 Not at all. You're the final call on the answer you find best suited to your question.

(disclaimer - Former USAF armament specialist)
The intention was not necessarily to "ram", but rather get eyeballs on the situation and take whatever action was needed. That may have, eventually, been a "ramming".
Obviously, that was not their first choice of action.
And ramming a 757 was not a thing that was practiced or planned for.
As far as " this was in the days before armed jets stood ready to take off at a moment's notice", that is false. There were 7 bases around the country, with fighter jets on alert. On that specific day, 2x F-15 from Otis AFB, MA and 2x F-16 from Langley AFB, VA had already launched to intercept the other airliners that were in play. Those 4 were fully armed.
Andrews AFB MD was not one of those few alert bases.
The pilots from the DC Guard knew that the WTC had been deliberately targeted, and would take whatever action that turned out to be needed.
For them being 'unarmed', that is the usual case for 99% of fighter jets on US soil. They don't all sit around with missiles and hot guns.
And to put missiles on them takes time. Time they didn't have. It would be a minimum of 30 minutes to get the missiles out of the storage area, tow them to the flightline, and load them on the jets.
Munitions are NOT stored right next to the jets.
While the DC Guard jets were being launched, other jets were being prepped with missiles. Turned out not to be needed.
The question often asked is how the USAF was caught with their pants down.
Well, the alert jets around the country were mainly for threats coming in from the coasts or over Canada. Russian Bears, for instance. Perimeter defense.
Not a known friendly civilian airliner, suddenly turning hostile once it is already inside the perimeter.
And up until the mid-90's, there were many more jets on alert around the country. East coast, probably every couple hundred miles. Loring ME, Griffiss NY, Langley VA, Shaw, SC, on and on all around the coast and borders.
The drawdown of the mid 90's did away with most of that tasking.

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Having armed fighter jets is one thing. What about land-to-air missiles? Weren't there any that could be used to shoot down that plane?

I'm sorry that some are frustrated that there aren't interesting links, but not everything is on the Internet.
The media loves to tell the story of the unarmed fighters ready to (presumably give their lives to) ram the jetliners.  But they actually have two weapons at their disposal.
TP rounds are real bullets
And peacetime fighters carry them for a variety of reasons.  However, they will only fire if the gun is mechanically and electrically enabled, which cannot be done from the cockpit - if the ground crew did not enable these, the cannon would be inoperative.
A bullet is just a chunk of metal moving fast.
A combat fighter-jet round is not a bullet; it’s a shell like an artillery shell, effectivly a flying hand grenade.  It includes a device called a fuze which makes it detonate on impact or flight time.  The bursting charge fires, blowing shrapnel in every direction.  Just what you need to down enemy fighters.  These jets didn’t have any of those.
The Vulcan cannon recovers its empty shell casings, so even a combat fighter that’s Winchester (out of ammo) has a lot more weight than one with no ammo at all. (again because of the brass) "No ammo" would be undesirable for weight and balance reasons, and why swap ammo in/out every time they go to the range?
They tend to fly with M55A2 "TP" which stands for "Target Practice" rounds, and are mixed with M220 "TP-T" Target Practice/Tracer" rounds. These are cheap, and don’t have the expensive fuze or the bursting charge. But they’re not blanks; they do have a “fast chunk of metal”. In fact, practice rounds used to be painted, so when they swapped paint with a practice target, it could be examined to see who shot the round.
A practice round will do a hell of a lot of damage to a non-jinking, subsonic jetliner. Simply putting a random hole in an airplane is unlikely to crash it; that’s why shells are needed in dogfights. But with a jetliner, the fighter pilot would have time to line up accurately on an engine and hit it with enough practice rounds to wreck it. (At small risk to shrapnel damaging the fighter, but the pilot can eject).
Or could focus on a control surface, such as the elevator or its operating mechanism.
Ramming
Ramming is not necessarily a suicide gesture.  It was often used as a tactic in war by pilots who had no intention of dying.
It is a matter of ramming sturdy points on your aircraft, such as weapons hardpoints, landing gear mounts, or even drop tanks; against fragile points on the other airplane.   Pilots are well familiar with these points.
Such an attack could be made at great advantage against a jetliner, which has no rearward visibilty at all, and could not anticipate the attack.

Also from comments (Thanks WPNSGuy):

Actually, the jets are generally loaded with 20mm TP. Weight and balance. Non explosive, non incendiary. But will fire just like combat level rounds. In the F-16, 510 rounds. The issue is the gun is hard wired safe by an external device, that can only be removed from outside the cockpit. In the interest of time on that morning, I do not know whether the ground crew removed that on those 2 jets.

[The safety lock] is not complex, but the access port is right next to the intake. If engine is already running, not a place to be taking things apart. And generally, it is safety wired.

And again, thanks WPNSGuy:

I can't find any specific published references for jets being loaded with 20mm TP all the time. But, in 16 years as an armament spec, 9 years specifically on F-16, I can't think of a single instance when a jet was loaded with non-fireable dummy rounds. Yes, fiberglass duplicates, weighted appropriately, exist. But we didn't fly around with them. And unless the jet was on a specific training mission for the gun, it was mechanically safed with the 'holdback tool', and/or electrically safed by having the actual power cable disconnected.

And thanks too to Charles Bretana, who says:

I have 2000+ hours in a USAF fighter (The F-4C/D/E Phantom). The F-4E had an internal gun. And we ALWAYS carried 640 rounds of 20mm TP ammunition. The only thing necessary to arm the gun was to pull a safety pin out of the gun assembly before we took off. Why you ask? Why not load "Dummy" rounds instead? Why? Then for any training mission the arming crew would have to offload the dummy ammo to load the TP. And Logistics would have to manufacture these dummy rounds and distribute them, Why do that when we already had thousands and thousands of TP rounds available?

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